A large tent filled with pungent fumes as hundreds of cars were stopped under it to check whether their tailpipes were emitting an excessive amount of pollutants.

Workers directed more than 460 vehicles to park in the Madera District Fairgrounds and then roll up to the tent to receive a free tailpipe probe, in which an emissions sampling probe is inserted into the tailpipe to measure the level of pollution the vehicle is emitting.

The Tune In and Tune Up event, organized by the San Joaquin Valley Air Pollution Control District, aims to help eliminate mobile source pollutants generated by older, “out of tune” cars by helping owners pay for needed repair work to get their vehicles “tuned up.” The program also serves to educate people about improving air quality in the San Joaquin Valley by reducing emissions, said Roger Teschner, event organizer for Valley Clean Air Now, which is a nonprofit advocacy group that sponsored the event.

Teschner said the number of cars signed up on Saturday nearly doubled since the last time the event was held in Madera in 2011. To qualify for the free tailpipe test, vehicles had to be registered to residents of the San Joaquin Valley, an eight-county highly polluted region in Central California that Madera is part of...